The present invention relates to telecommunications networks, particularly to fiber optic transmission of broadband signals, including voice telephony, in a single network.
Such systems are rapidly being installed worldwide because of fiber optic's advantages as a transmission medium, greater bandwidth and noise characteristics, as well as cost when compared to metal conductor based media. They allow the distribution to customers of both voice grade telephony and broadband services, such as CATV over the same network infrastructure.
Examples of such systems are discussed for example in an article by Don McCullough of Broadband Technologies, Inc., Research Triangle Park, N.C., entitled The Raging Technology Debate: Hybrid Fiber/Coaxial Cable vs. Fiber to the Curb, Lightwave Magazine, July 1995, pp. 36, 39, and 41.
The network types compared by McCullough, Hybrid Fiber Coax (HFC) and Fiber to the Curb (FTTC), are alike in that they employ fiber optic transmission between a central location and an intermediate node, where signals are interconverted betweeen optical and electrical form so that further transmission to and from customers is electrical. The systems differ in the distance of fiber transmission: For HFC, the fiber transmission links extend only to nodes that serve clusters of 500 to 2500 customers, while, for FTTC, the fiber links extend further, to clusters of 8 to 24. Because of this difference, the FTTC system can employ lower-cost twisted copper pair wiring between the intermediate node and the customer, while the electrical transmission medium used in the HFC system is coaxial cable.
The networks discussed by McCullough are also alike in that separate fiber links are employed for the analog radio-frequency (RF) television signals that are broadcast to customers and for the telephony signals. The analog fiber links are unidirectional, while the telephony links are bidirectional. The telephony signals are digitally encoded and multiplexed over fiber links that include optical modulators and detectors that are more expensive than those used for analog video. For the HFC system, the digital telephony signals are also transmitted over the coaxial cable portion of the network bidirectionally by digitally-modulated RF carriers.
Another type of broadband system that combines the transmission of telephony and analog video is disclosed by Beierle et al. in U.S. Pat. No. 5,351,234. This system has a basic advantage over the systems described above in not requiring any signal processing at an intermediate node in the network. If desired, the subscriber interface unit could serve a cluster of subscribers, but it need not serve more than one.
As therein described, this system employs bidirectional transmission on coaxial cable; it uses the same analog RF modulation technique as broadcast analog video. As shown by the invention such systems may advantageously be adapted to include at the central location fiber-optic links of the sort used for video in the systems discussed by McCullough; these would of course have to be bidirectional. This adaptation would introduce an intermediate node into the network, but no telephony signalling or multiplexing function would be performed there, only simple optical amplitude modulation and demodulation. It has the advantage over the HFC systems described above in not requiring separate fiber links for telephony. The reduced complexity leads to lower cost.
Nonetheless, as telecommunications becomes more competitive, it remains important to further reduce costs, particularly in all-fiber networks.